Test piece for inspection penetrant performance assessment and comparison

ABSTRACT

A test panel and process of preparing a test panel for evaluating the performance of inspection penetrants by inducing cracks into a brittle coating. The process includes drilling one or more holes from the back side of a test plate which has been plated on a plated side. The holes are drilled partway through the test plate and create cavities. A penetrating tool which has a point is inserted into a cavity and the point is pressed in a slow measured manner from the back side of the panel which has a brittle plated coating on the front side to be cracked. The result is a series of cracks formed in an area opposite the area of the cavity.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to applicant's application Ser. No.09/197,884, filed Nov. 23, 1998, by the present applicant and which hasissued as U.S. Pat. No. 6,311,538.

BACKGROUND OF THE INVENTION

Inspection penetrants are used to find surface cracks in all types ofinsoluble materials, but primarily in nonmagnetic metals, such asaluminum, high nickel alloy, stainless steel and titanium. These metalspredominate in the aerospace industry. On many aerospace parts, such asrotating discs of turbine engines, even the most microscopic cracks mustbe found, as these microscopic cracks will propagate under the stressesand loads placed on them during operation. Propagation can be extremelyrapid and part failure sudden and catastrophic.

Inspection of such critical parts for surface cracks, whether newlymanufactured or parts that have been in service, is accomplished withfluorescent penetrants. The process consists of application offluorescent penetrant, an oily-like fluid charged with fluorescentdyestuff, to the surface of the part by immersion, brushing or spraying;allowing the fluorescent penetrant to remain on the surface forsufficient time to enter microscopic surface openings by capillaryaction; removing the penetrant that did not enter cracks and surfaceflaws from the surface by washing with a pressure rinse of plain wateror with the aid of a detergent or emulsifying agent. The part is thendried in an oven and typically a developing agent is applied to bringthe crack entrapped fluorescent penetrant back to the surface so it maybe seen as a glowing line under ultraviolet light (black light) in adarkened room.

The fluorescent penetrant system must function properly or cracks willgo undetected. These cracks can propagate into catastrophic failures.For example, if the penetrant has become contaminated with differentsubstances, its fluorescing properties will be diminished. If themechanism for applying the developing agent is malfunctioning, the flawswill not be clearly identifiable under the ultraviolet light. There areother causes for failure such as rinse water with a too-high temperatureand a too-high concentration of detergent which will prematurely removepenetrant from cracks.

To defend against processing critical aerospace and turbine engine partsin a malfunctioning fluorescent penetrant system, a general requirementis to prove the system each day before it is used by processing a testpanel with known defects. If, after processing, these known defects arenot displayed as anticipated, i.e. with the same completeness andbrilliance as in previous tests, this alerts the operator of the need tocheck the system for a malfunction.

Probably the best known panel for this purpose, and most widely used, isthe PSM-5 panel manufactured by Sherwin Incorporated, South Gate,Calif., to Pratt & Whitney drawing TAM 146040, which drawing is datedApril 1975. The PSM-5 panel is a 4×6″ piece of stainless steel,thickness 0.090″ with a strip of hard chrome plate running lengthwisedown one side. The thickness of this chrome plate is 0.003″ or greater.As hard chrome plating is applied electrolytically, its thickness willvary over the surface with heavier coating to be anticipated at theedges. Five cracks of varying magnitude, evenly spaced, are induced byexerted pressure opposite the hard chrome strip with a Brinell hardnesstest instrument. The balance of the front of this test piece andadjacent the chrome strip is a rough area obtained by grit blasting withaluminum oxide or other media.

The most difficult task in manufacturing the PSM-5 panel is in theformation of the smallest crack, a crack diameter in the range of 0.015″to 0.030″. This small crack, necessary to verify the system's ability tofind the truly microscopic crack, is difficult to produce, as there isno room for the slightest error in the plating composition or in thepressure exerted by the hardness tester. The crack is formed by pressureexerted by a hard round ball on the side opposite to the plating. Theplating side is backed up against a selected surface. The pressure ismeasured by weight, pounds or kilograms. Although the hardness testequipment includes an instrument to indicate in kilograms the weightforce, there is a lag in indication and this equipment is notsufficiently precise to give controlled crack formation in the area ofsmall cracks whose detection is required by today's advanced industry.

The method of inducing cracks with a hardness tester has an inherentdeficiency. If the induced crack fails to meet its specification, it isnot possible to review and remeasure to verify if the prescribedpressure was applied. The indentation left by the hardness tester is notmeasurable with ordinary tools.

As the aerospace industry continues to reduce the weight of theirvehicles while at the same time demanding higher performance and placinggreater stress loads on the components, the need for the penetrantprocess capability to locate smaller surface flaws becomes morecritical. The need for a test piece that verifies the inspectionsystem's capability to meet these more demanding specificationrequirements is obvious.

No two PSM-5 panels are exactly alike. They are produced individually.Further, the cracking method cannot be precisely controlled. It is notpossible to hold two PSM-5 panels side-by-side and expect to seeequivalency in the crack patterns. The PSM-5 panel cannot be used toreliably compare relative sensitivity between two different penetrantsbecause no two panels are equivalent. One cannot expect to process onepanel with new penetrant and another with in-use penetrant and obtain ameaningful comparison. This is due in part to the fact that the panelsare produced separately and not as one piece. It is also due to themethod of cracking, applying pressure until cracking occurs.

Recognizing this deficiency in the PSM-5 panel, governing agencies nowrequire the user of the PSM-5 panel to photograph the panel when firstprocessed with unused penetrant materials in the laboratory and, then touse this photograph to compare results obtained when the panel issubsequently processed through the production penetrant system on adaily basis. Although this has some utility, it is not trulysatisfactory, since, in order to take photographs under ultravioletlight, time exposures are required and the photographed fluorescentcrack indications will vary in size and definition with exposure time,as well as with film negative and printing paper and technique. Furtherthe photograph must be viewed under white light, as the crackindications in the photograph do not fluoresce. The panel itself must beread under ultraviolet light. Such indications cannot be viewed andcompared to actual fluorescing indications in the darkened inspectionbooth where the lighting is ultraviolet and be meaningful.

But this recently imposed requirement of a photographic comparison insuch specifications as ASTM E 1417, despite its inadequacies, isevidence of the need of a reference point when interpreting the panel'sresults.

Also, although the PSM-5 cracks may be small, e.g. 0.015″ of an inch indiameter, their depth is the depth of the plating which is 0.003″ orgreater. The nature of the hard chrome requires a plating thickness ofclose to 0.003″ to crack with a pressure load, otherwise, it stretchesuntil it splits uncontrolled. A crack 0.003″ or more in depth retainsconsiderable penetrant and, therefore, does not reveal abusiveover-washing and over-heating in processing as readily as it should.Further, a crack with this depth tends to retain penetrant even whensubjected to extensive cleaning between tests. Such retained penetrantfrom a previous test leads to erroneous conclusions on subsequent tests.ASTM E 1417 stresses the need for adequate cleaning of the known defectstandard between tests.

To meet today's need, the brittle coating must lend itself to controlledcracking in thin coatings, less than 0.002″. Thin coats of chromeplating, such as 0.001″, do not crack uniformally even though pressureexerted is uniform. Although there may be a place for chrome plating,metal conversion coatings, silicate and other brittle coatings, andthese coatings lend themselves to the controlled method of cracking thatI discovered, my favorite brittle coating is nickel plate, eitherelectrolytic or electroless nickel type, because a thin coat of nickelplate lends itself to controlled cracking. Cracks can be induced with myinvention in nickel plate in a coating as thin as 0.00025″ in acontrolled manner.

Although we have fabricated panels with plating thickness of 0.00025″which have proven to be a practical tool, typically, we have found theplating thickness of 0.001″ the most useful. Cracks of this depthduplicate the small shallow cracks which are the object of today's mostexacting inspections. The penetrant retained in a crack reservoir 0.001″deep with a diameter of 0.015″ is sufficiently minuscule for theentrapped penetrant to be affected deleteriously by over-washing,over-heating and inadequate developer application. Abusive processing ora substandard penetrant material is more readily apparent with a testpiece with shallow, 0.001″ cracks than when crack depth of the testpiece is 0.003″ or greater.

An added advantage of the thinner coating is the ease of cleaning thepanel and freeing the crack reservoirs of materials deposited duringprevious use. A 15 minute soak in alcohol is all that is required toclear the reservoirs whereas a lengthy bath in an ultrasonic cleaningtank charged with a chlorinated solvent often has to be repeated toclear the crack reservoirs of the PSM-5 panel.

Like the PSM-5 test piece, my favorite substrate is stainless steel, asit is rugged in construction, not subject to corrosion, and withstandsthe rough handling when sent through the penetrant system.

There are other test pieces used to evaluate inspection penetrants, suchas thermally cracked aluminum blocks and “nickel-chrome” test panels.These panels are not suitable replacements for the PSM-5 type panel andare not intended for use in evaluating the functioning of an inspectionpenetrant system.

The nickel-chrome test panel for the sake of clarification should bedescribed, so it will be understood that it does not compete with ourinvention. It is known in the industry as the “twin nickel-chromepanel.” This panel's substrate is brass, subject to corrosion. Theplating is brittle nickel with a flash of chrome. Panels usually measureabout 1¼×4″ and are 0.06 thick. The plating will vary in thickness fromas thin as 0.0002″ (5 μm) to 0.002″ (50 μm).

The panels are bent over curved anvils to induce cracking and thenstraightened, as shown in U.S. Pat. No. 4,610,157. The main difficultywith the “twin nickel-chrome panel” is the form of the cracks, straightcracks, running laterally from one edge of the test piece to the otheredge. The cracks' geometries are open troughs. Penetrant that entersthis type of cracking is easily flushed through the open trench ortrough. While it is possible to compare the visibility of two penetrantsby judicious removal of surface penetrant, it is not possible to processthese test pieces in the work environment. The penetrant flushes fromthe “troughs” too readily. These test pieces have a limited function.For this and other reasons, they are not a practical tool for monitoringa penetrant system. The “twin nickel-chrome panel” is primarily alaboratory tool. Another patent which shows a test panel process is U.S.Pat. No. 4,078,417.

The nickel-chrome test panel is normally produced as a single panelwhich is later sheared into matching panels. It can be used forside-by-side comparisons of different penetrant materials in thelaboratory. It is not sufficiently rugged to be used to measure thecapability of a production penetrant system and it gives erroneous datarelative to the wash cycle.

The present continuation in part application is concerned with the meansto further control the magnitude of the induced cracks made in the testpiece. The objective of the improved process is to manufacture testpieces with precise crack sizes, both in crack depth and crack length.The use of the penetration method described in the present applicationhas made possible precision induced cracking in the production ofvirtual twin test pieces where a single brittle coating specimen isshared into equivalent sections as the brittle plating coating isapplied at the same time on both sections. However, it has beendifficult or impossible to extend this control when the plating differs,which is the case with specimens plated at different times. The presentimprovement makes possible such depth and length control by confiningthe cracking force to a predetermined limited area by weakening thesubstrate to such a predetermined limited area.

SUMMARY OF THE INVENTION

It was discovered that by drilling holes part way through the substratefrom the back of the substrate toward the plated side, cracks could beinduced in a much more controlled area, namely, the area equal to thediameter of the hole which has been drilled part way through thesubstrate. The tool used to produce cracking described in the presentapplication is inserted in the incompletely drilled holes and forcedagainst the bottom of the hole an amount sufficient to create cracks inthe plated surface on the plated side of the test plate. The result wasa highly controlled crack depth and length which did not require theplating of other panels to be done at the same time.

The present invention is for a method of forming controlled cracks in apredetermined area of brittle plating on a plated surface of a testplate which comprises a substrate which has a plated area on a platedside and a back side. One or more holes are drilled part way through thesubstrate from the back side of the test plate opposite a plated area ofthe test plate to create a cavity having a cavity diameter and a cavitybottom. The point of a hardened tip is inserted into the cavity andforced against the cavity bottom with sufficient force to create adeformation of a portion of the plated area beneath the cavity bottom.This forms a plurality of cracks in the plated area limited to an areano larger then the cavity diameter. Preferably, holes of different sizesare drilled in the test plate to provide cracks of different depth andlength. The holes are preferably drilled more than half way through thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the tool used in the process of thepresent invention together with a test panel being formed.

FIG. 2 is a front view of the plated side of a test plate made accordingto the process of the present invention.

FIG. 3 is a front view of the plated side of the test plate of FIG. 2sheared into two individual test plates.

FIG. 4 is a plan view of a test plate as viewed from the back side withtwo sets of three drilled cavities therein.

FIG. 5 is a cross-sectional view taken along line 5—5 of FIG. 4.

FIG. 6 is a bottom view of the test plate of FIG. 4.

FIG. 7 is a cross-sectional view taken along line 7—7 of FIG. 5.

FIG. 8 is a cross-sectional view taken along line 8—8 of FIG. 5.

FIG. 9 is a cross-sectional view taken along line 9—9 of FIG. 5.

FIG. 10 is an enlarged cross-sectional view of a cavity with a hardenedtip inserted therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The tool of the present invention is shown in FIG. 1 and indicatedgenerally by reference character 10. The tool has a frame 11 whichsupports a block 12. Block 12 holds a manually turned penetratingassembly 13 which has a handle 14, a threaded shaft 15 and a penetratingtool point 16. A finger 17 moves with the threaded shaft 15 and thepenetrating point 16. Finger 17 presses against the measuring arm 18 ofdial test indicator 19. Dial test indicator 19 is held by an arm 20which is secured to frame 11. Frame 11 also holds a backing block 21.Backing block 21 has a cavity into which an anvil (not shown) is held.

The important feature of the process of the present invention is tocontrol the depth of the penetration of point 16 into the test plate orpanel 25. Panel 25 is preferably stainless steel including 316, 321 and18-8 and panel 25 has a plated side 26 and an uncoated side 27. Theplating can be formed by hard chrome, electrolytic nickel andelectroless nickel to a thickness of between 0.0002″ and 0.002″. Thisplating forms a brittle coating in which cracks form when the backsurface is penetrated by point 16. By moving the point 16 into contactwith the uncoated side 27, zeroing the dial test indicator 19 andmeasuring the distance of penetration, a highly controlled series ofcracks is formed. For example, penetration to a depth of 0.016″ istypical to make a crack with a diameter area of 0.015″. Most commonly,five craters such as craters 29, 30 and 31 of FIG. 1 are formed ofincreasing depths equally spaced along panel 25.

The industry also requires a test piece for comparing penetrantsensitivity. Such panels are shown in FIG. 3 and are formed from asingle panel shown in FIG. 2. In the past, twin nickel chrome panelshave been made by plating a brass panel which is then bent over an arborwhich induces lateral cracking. The plated brass is then straightenedand sheared into two equivalent pieces. Because these cracks runlaterally, they are in effect troughs which are open at both ends. Suchopen troughs tend to be too easily flushed by normal processing andthus, have limited practical function. The test panels of the presentinvention do much more to monitor the system in that they comparevisibility after production-type processing, they compare washabilitywith the grit-blasted roughened area, and provide an accurate controlduring production. These panels are made preferably using an electrolessplated primary nickel, but with a higher percentage of phosphoric acidthan normal. The addition of phosphoric acid gives added brittleness tothe coating making it possible for crack plating with a thickness asthin as 0.0002″ (5 micrometers). The point 16 is preferably round ratherthan a depressed oval as formed by a Brinell hardness tester.Furthermore, the method of inducing cracks of the present inventionpermits the accurate reinsertion of the needle-like instrument thatinitially formed the round hole if it is necessary to increase themagnitude of the cracking. In the past, there has been considerabledifficulty in producing this smallest diameter crack, the one measuring0.015″ or less.

As shown in FIG. 2, panel 30 is a rectangular plate having a top edge31, a bottom edge 32, a right side 33 and a left side 34. Panel 30 haspositioned midway between the right side and the left side. A firstmiddle zone 36 is located between vertical center 35 and right side 33,a second middle zone 37 is located between vertical center 35 and leftside 34. The area between the first and second middle zones 36 and 37has been grit-blasted and a pair of support holes 38 and 39 have beenformed one-fourth way in from the right side and the left side. The areabetween the first middle zone 36 and right side 33 and second middlezone 37 and left side 34 are simultaneously plated, preferable withelectroless plating, primarily nickel, but with a high percentage ofphosphoric acid. Then a series of increasing cracks 40, 41, 42, 43 and44 are formed in the brittle plated area 50. Similarly, cracked areas51, 52, 53, 54 and 55 are formed by the above-described process inbrittle plated area 51.

Next, as shown in FIG. 3, the panel 30 is sheared into two panels 52 and53 which are nearly identical since they were plated at the same timefrom the same sheet. These panels, therefore, become very useful in thetwin panels test.

A panel 60 is shown in FIG. 4 from the back side. Panel 60 has twoplated areas 61 and 62 shown in FIG. 6 and indicated by phantom lines inFIG. 4. Three cavities comprising a smallest cavity 63, intermediatecavity 64, and a largest cavity 65, are drilled part way through panel60 in an area adjacent plated areas 61 and 62. These cavities are shownin cross-sectional view in FIGS. 7, 8, and 9. For instance, in FIG. 7 itcan be seen that hole 63 passes through a majority of the substrate 66,but does not enter the plated area 67. For a substrate of {fraction(3/32)} thickness and a plated area of between 0.001 and 0.002 inchesthick, hole 63 passes a depth of about 0.007 leaving a thickness ofsubstrate and plating between 0.022 and 0.025 inches.

As shown in FIG. 10, a hardened tip 68 having a point 69 has beeninserted into cavity 65 and forced downwardly against backing block 70.This forms a cracked area 71 having a required diameter controlled bythe diameter D of cavity 65. Cracked areas 71, 72, and 73 are shown inFIG. 6 and correspond approximately to the diameter of the cavities 65,64, and 63, respectively.

Because of the accurate limitation of the resulting crack depth andcrack length, the test plate can be created which provides a veryreproducible result as a standard. The hole gives better control of thecrack pattern. Without the hole, the crack can get too big. With thehole, the crack pattern generally does not exceed the hole diameter.

The present embodiments of this invention are thus to be considered inall respects as illustrative and not restrictive; the scope of theinvention being indicated by the appended claims rather than by theforegoing description. All changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

I claim:
 1. A method of forming controlled cracks in predetermined areasof a brittle plating of a test plate, said test plate having a substratewhich has a plated area on a plated side and a back side, said methodcomprising: drilling a hole partway through said substrate from the backside of said test plate opposite an area which is plated on the platedside thereby creating a cavity having a cavity diameter and a cavitybottom; inserting a point of a hardened tip into said cavity; forcingsaid point against said cavity bottom with sufficient force to create adeformation of a portion of said plated area beneath said cavity bottomthereby forming a plurality of cracks in said plated area limited to anarea no larger than and controlled by the cavity diameter.
 2. The methodof forming controlled cracks of claim 1 wherein a plurality of holes ofdiffering diameters are drilled partway through said substrate and apoint of a hardened tip is inserted into and forced against theresulting cavity bottoms to form a plurality of cracks at locationsopposite each of said plurality of holes.
 3. The method of formingcontrolled cracks of claim 1 wherein said point of said hardened tip isforced a predetermined distance against said cavity bottom.
 4. Themethod of forming controlled cracks of claim 1 wherein said substratehas a thickness of about three thirty seconds of an inch thick and saidhole is drilled more than half way through said substrate.
 5. The methodof forming controlled cracks of claim 4 wherein holes of at least threedifferent diameters are drilled partway through said substratecomprising a hole of the smallest diameter, an intermediate diameter andthe largest diameter.
 6. The method of forming controlled cracks ofclaim 5 wherein the hole of the smallest diameter is drilled furthestinto said substrate, the hole of intermediate diameter is drilled anintermediate distance into said substrate and the hole of largestdiameter is drilled the least distance into said substrate.
 7. Themethod of forming controlled cracks of claim 6 wherein the hole of thelargest diameter is drilled about half way through said substrate.